Attenuated viral strains are usually obtained by multiple passages in specific cell lines or by cold-adaptation or by genetic manipulation in recombinant viruses. Attenuation of viruses may be the result of a single nucleotide change in a critical gene product and hence has the possibility of reverting to the wild type (WT) phenotype due to genetic instability. The ability to induce interferon(IFN) synthesis in the host and the sensitivity to the antiviral activity of interferon are characteristic features for a given virus and hence may be used to characterize WT and attenuated strains that are derived from the parent. Such information on RSV is lacking and in this project, our goal is to define these characters of WT and three temperature-sensitive (Ts) mutants of RSV subgroup A or. In addition, the role of IFNs in virus attenuation is investigated. Human lung cells, A549 and MRC-5 cell lines are infected with the wild type (WT) RSV strain A2 and at different time points post infection, samples were collected to analyze for the presence of different IFNs for antiviral activity,IFN-specific mRNA, or protein. The most attenuated vaccine strain produced the most antiviral activity, IFN-b mRNA and protein compared to its wild type parent strain at permissive temperature. But at restrictive temperature, no IFN was induced due to lack of replication. Hence IFN induction does not seem to contribute to the attenuation of this mutant virus. Similarly, the effects of various IFNs on RSV replication is currently investigated in both MRC-5 and A549 cells. Cells were pre-treated with IFNs a, b and g, at 0 - 10,000 u/ml conc.for 20-24 h and infected with RSV A2, RSS-2, RSV B1 or other mutant strains. The sensitivity of these viruses to IFN was determined in terms of reduction in virus yield, RNA replication or viral protein synthesis. As a control virus with known sensitivity to IFN, human Parainfluenza Virus type 3 (hPIV3) is used and similar parameters are evaluated. Our results indicate that while hPIV3 is accutely sensitive to IFNs a and b (> 3-4 log10 reduction in virus yield), RSV A2 is highly resistant (~0.5-1.5 log10 reduction in virus yield) to the antiviral activities of various IFNs. This was also confirmed in cell lines stably expressing human MxA protein, an IFN-inducible protein associated with antiviral activity against certain viruses. While hPIV3's growth was found to be sensitive, RSV A2 seem to be normal in these cell lines. These studies indicate that wild type RSV A2 is resistant to IFNs and might have developed mechanisms that counter act the antiviral activities of IFNs. Further studies with attenuated, mutant viruses also demonstrated that RSS-2 or its mutants Ts1A, 1B or 1C and RSV B1 or cp-52 mutant were all similarly resistant to the antiviral effects of type I IFNs. This data indicate that neither increased IFN induction or enhanced sensitivity of the mutant viruses contributes to their attenuation.